1. Field of the Invention
The present invention relates to a light source drive circuit, an optical scanning apparatus, and an image forming apparatus.
2. Description of the Related Art
In a related-art image forming apparatus for use in product printing, etc., a predetermined optical output is obtained from a light source such as an LD (laser diode), etc., to expose a photosensitive body therewith and express a density of an image.
Now, it is known that, in the related art, a light emission delay time occurs, which depends on a response characteristic of a light source before obtaining a predetermined optical output from the light source. Moreover, in the related art, it is known, for example, that, from a time at which a drive current is supplied to a light source to a time at which an optical output is detected, a light emission delay time occurs which depends on a parasitic capacitance of a circuit, etc., in which the light source is mounted.
Therefore, in the related-art image forming apparatus, when a time to cause an optical output is set to a short time of less than or equal to a few ns, for example, the optical output becomes less than a predetermined light amount, and the density of an image decreases, possibly causing unevenness in the image.
Thus, in the related art, schemes are provided to solve the above-described problems. For example, Patent document 1 discloses providing a charge and discharge circuit, wherein an overshoot current is generated by discharging at a time of a rise of an output of the LD (laser diode) to reduce a light emission delay time which depends on a response characteristic of a light source. Moreover, Patent document 2 discloses initially superposing a threshold current at a start time of lighting the to and controlling a light emission amount thereof. Furthermore, Patent document 3 discloses a method to output, prior to a drive current, a current over a current threshold of a light element for a shorter time period than a time period to output the drive current.
However, the overshoot current in Patent document 1 is generated primarily for reducing a delay time which depends on the response characteristic of the light source, so that it is difficult to improve the light emission delay time which depends on a parasitic capacitance. Moreover, in Patent document 2, while the threshold current is initially superposed at the start time of lighting the LD, the threshold current is insufficient for charging to overcome a circuit parasitic capacitance, so that it is difficult to sufficiently reduce a light emission delay time which depends on the parasitic capacitance. In particular, it becomes more difficult to reduce a delay time which depends on the parasitic capacitance in a circuit with a large parasitic capacitance and a light source with a large differential resistance. Furthermore, in the configuration disclosed in Patent Document 3, which supplies the current prior to the drive current, it may be difficult to optimize light output characteristics of the light source.
At least one embodiment of the present invention is aimed at resolving the above problem and providing a light source drive circuit, an optical scanning apparatus, and an image forming apparatus which make it possible to reduce a light emission delay time of an optical output and to improve a response characteristic.